The present invention relates to a flexible pipe, and more particularly to a flexible pipe having integrated end fittings. Its structure is particularly, but not necessarily, adapted to an application in the oil industry for transporting liquids or gases.
Such flexible pipes are generally relatively short, being of a length that rarely exceeds about a dozen meters, and they enable flexible connections to be made between fixed pieces of equipment. When lengths longer than the available lengths are needed, then a plurality of flexible pipes can be connected together.
These flexible pipes, in particular in an oil industry environment, can be subjected to very high levels of stress. The pressure of the effluents they transport can exceed 1000 bars (i.e. 100 megapascals (MPa)) and their temperature can exceed 150° C. The effluents may be constituted by liquids or gases that can be highly corrosive, such as oil, aromatic liquids, water, hydrogen sulfide (H2S), or carbon dioxide gas (CO2). Such pipes can be subjected to a wide variety of external stresses, including in particular traction on their ends during assembly, exposure to temperatures that can lie in the range −40° C. to +70° C., and abrasion against the ground.
Their lifetime is variable depending on the application to which they are put, but in general it should be greater than 10 years, and by construction they must be capable of guaranteeing safety for personnel and equipment. They must be light in weight in order to facilitate transport and handling. They must be fitted with protection means for avoiding damage that would be created by a radius of curvature that is too small.
In the description below, the term “flexible pipe” designates an assembly made up of a “pipe body” (supple cylindrical tubular main portion) together with two connector end fittings, one being fitted to each of the ends of said main portion. In cross-section, the pipe body is generally constituted, going from the inside towards the outside, by a “carcass,” a “liner,” optionally a “drainage layer,” a “reinforcement layer,” optionally one or more “traction layers,” and an outer “cover.”
The carcass is generally constituted by a shaped metal strip fastened to form a continuous cylinder. Its function is to prevent bubbles forming in the liner in the event of sudden decompression (the so-called “blistering” phenomenon). In addition, it serves to avoid the pipe body collapsing when an external pressure is applied that is greater than the internal pressure. Finally, it serves to absorb the axial loads applied to the pipe by preventing its diameter decreasing, and thus preventing the pipe body from being damaged. This carcass must be sufficiently supple to ensure that the pipe has the required degree of flexibility.
The “liner” is generally made of thermoplastic material or of elastomer. Its function is to provide gas- and liquid-tightness from the inside to the outside and ensures internal fluid integrity. This layer must have a small swelling coefficient (generally less than 10%), and also low permeability.
Where it exists, the “drainage layer” serves to drain any gas that diffuses through the liner to vents that are situated at the end fittings of the flexible pipe. They thus serve to avoid any blisters or bubbles forming in the outer protective sheath.
The “reinforcement layer” withstands the pressure developed by the fluid on the liner, and is generally constituted by a helical winding of one or more crossed layers.
These layers may either present a pitch that is short (i.e. a winding angle relative to the axis of the pipe that is close to 90°), or else a pitch that is long, being close to the equilibrium value for the structure when it is subjected to internal pressure, namely 54.7°.
The “traction layer(s),” where present (often in the form of two crossed sheets), is/are generally constituted by helically winding reinforcement that is wound at a long pitch with a winding angle relative to the axis of the pipe that lies in the range of 20° to 50°, serving to take up the axial forces exerted on the pipe. These layers may be situated inside or outside the reinforcement layer.
The outer protective cover is made of thermoplastic material or of elastomer. Its function is to protect the pipe from external attack (water or salty conditions, for example).
It should be observed that a distinction is drawn between two categories of flexible pipe, those that are said to be “bonded” in which the various above-mentioned layers are bonded to one another, and those that are “not bonded” in which the various layers are independent.
The subject matter of the present invention is more particularly suited to bonded flexible pipes, however it can also apply to non-bonded pipes.
The end fittings serve to enable the pipe to be connected to various pieces of equipment. They are subjected to high levels of stress, and they must present mechanical strength that is not less than that of the pipe body.
The pipe body may be manufactured in great lengths using a continuous fabrication process, relying mainly on thermoplastics, or in shorter lengths, generally of 6 meters (m) to 12 m, using a method of manufacture on a support rod and often requiring vulcanization, relying mainly on elastomers as their materials.
Very generally, for all these types of flexible pipe, the winding pitch of the traction layers is identical over the entire length of the pipe and the end fittings are assembled thereto after the pipe body has been fabricated. In order to make such flexible pipes, it is therefore necessary initially to cut the pipe body to the desired length, and then to secure the end fittings thereto.
It should be observed that at the connection between the pipe body and an end fitting, there is an interruption in the reinforcement layer and in the traction layers. It is therefore necessary to connect them to the end fittings using suitable mechanical connection methods, e.g. by crimping, wedging, and/or adhesive bonding, suitable for passing stresses from the pipe body to the structure of the end fittings.
In addition, it is necessary to make leak-tight connections between the end fittings and the liner of the pipe. These connections are particularly important since perfect leak-tightness must be maintained throughout the lifetime of the flexible pipe. Unfortunately, these connections must naturally withstand the same stresses as are withstood by the main pipe body, in particular in terms of pressure and temperature.
It will be understood that making such end fittings is difficult and their design raises serious difficulties, associated firstly with the magnitude of the mechanical forces involved, and secondly with the problem of maintaining sealing. The traction force generated by the internal pressure can be very high. By way of indication, it can reach a value of the order of 55,000 decanewtons (daN) for a pipe having a diameter of 100 millimeters (mm) and that is subjected to an internal pressure of 700 bars. To this force, it is necessary to add traction forces that depend on the application (mounting a suspended pipe, for example). Extremely high levels of stress are thus applied to the system for retaining the traction layers (often cables or metal section members) on the end fittings. The retention system must thus be very reliable, and must be capable of being subjected to a very large number of loading and unloading cycles (several million cycles) without damage.
With reference to sealing, this is achieved by the liner pressing against the reinforcement layer, and the leak-tight connection with the end is very often achieved by adhesive or by pinching. To maintain sealing, it is therefore necessary for the adhesive or the elasticity of the liner (for maintaining the pinching force) to continue throughout the lifetime of the pipe. End fittings suitable for satisfying these conditions are heavy and they often need to be assembled manually, with the help of appropriate mechanical means that are expensive.